Monthly Archives: November 2010

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In natural communities, each species is embedded in a web of interactions with other species—predators, prey, competitors, mutualists, and parasites. The effects of all these other species combine in complex, unpredictable ways. I recently discussed a study of protozoans living inside pitcher plants that found predators and competitors can cancel out each others’ evolutionary effects. Now another study finds that parasites and predators can interact to make desert-living gerbils adopt less effective foraging strategies [$a].

Allenby’s gerbil is a small desert rodent native to Israel’s Negev Desert. They make a living foraging for seeds, which might seem simple enough—but for small desert mammals, it’s a constant balancing act. Foraging requires continuously judging how profitable it is to continue gathering seeds in one spot compared to looking for another, maybe better, spot; and all the while watching out for predators.

The red fox—a major threat if you’re a tiny rodent, but hard to watch for when you’re scratching fleas all the time. Photo by HyperViper.

For small mammals, parasites like fleas can impose a real physiological cost—but they might also cause irritation that interfere with effective foraging. This idea led a group of Israeli reserachers to experimentally infest captive gerbils with fleas, and release them into an enclosure with a red fox.

It’s okay—the fox was muzzled! The research group was interested in how effectively the gerbils foraged in standardized patches of resources (trays of seed mixed with sand) in the presence of predators, and how being flea-ridden changed that foraging behavior. As metrics of foraging efficiency, they recorded how rapidly the gerbils gave up foraging in a single tray before moving on to another, which approximates how many seeds they left behind.

With no fleas, gerbils spent slightly—but not significantly—less time foraging in a single tray when a fox was in the enclosure with them. But gerbils infested with fleas moved on to a new tray substantially faster in the presence of a fox, leaving behind more seeds in the process. The study’s authors suggest that this is because the irritation caused by fleas distracted the gerbils too much to keep watch for a predator and forage at the same time—so flea-ridden gerbils made up for being less watchful by moving between patches of resources more rapidly.

So for gerbils, the presence of a second, different kind of antagonist amplifies the effects of a nearby predator. Fleas and foxes aren’t just a double whammy—the effects of both together are worse than the sum of each individually.

Well, I’ve successfully run the Seattle Marathon. I finished in 3 hours, 31 minutes, and 27 seconds, which is a pretty nice improvement over last time, in Portland.

Coming down the home stretch. Photo courtesy my cousin Adrienne.

I’ve had a nice hot shower, and my cousin—with whom I’m staying while in Seattle, and who came to watch me cross the finish line—keeps pressing protein-and-vitamin recovery shakes on me, so I’m actually feeling pretty good. It’s great end to a great fall break in Vancouver and Seattle. I took a lot of photos—there’s a slideshow after the jump.

To be fair, science is pretty difficult. The leadership of the American Anthropological Association is moving to remove references to science from the organization’s mission statement. (Fetishes I Don’t Get)

And finally, Robert Krulwich narrates a beautifully animated short film about an enduring mystery of human behavior: our inability to walk in a straight line without help from visual cues.

I’m spending a significant chunk of my Thanksgiving break in Seattle, for the purpose of running what will be my second marathon this weekend. Running, like cooking, is helping to keep me sane in the midst of teaching labs, finishing my dissertation research, writing said research up for publication, and trying to sort out what happens after my committee decides I’ve earned a handful of extra letters after my name.

Me at about mile 17 in last year’s Portland Marathon. I’m not quite dead yet.

My first marathon was last year’s Portland Marathon. Prior to 2009, I’d never run a race longer than five miles, but then that spring I let friends talk me into a half-marathon, and after running more than 13 miles, 26.2 suddenly didn’t seem quite so insane. Even so, training up for Portland was more than enough to make me realize that running what was (for me) a 3 hour-45 minute course is not really the same thing as running eight or nine 5k’s in a row.

Feed me!

I can make it through even a half-marathon on a good breakfast and carefully-judged pre-race hydration, but to go much longer I need more food (and water) mid-run. The long-term exercise involved in a long race is fueled by a combination of fat reserves and glycogen stored in the liver and muscle tissue. Glycogen is the more efficient fuel, so as exercise intensity increases, muscles draw on it more heavily.

If his muscles runs out of glycogen, a runner “hits the wall,” and may be forced to stop running altogether. I’ve done this a few times on long training runs, and it’s not pleasant—I’d end up all but walking the last couple painful miles. How long I can go before I hit the wall depends on my glycogen reserves, which in turn depend on the muscle mass in my legs—but it also depends on how fast I’m running, since glycogen use increases with effort. A computational study of the interactions between exercise intensity and glycogen consumption suggests that my first marathon time, 3:45, was close to the upper limit of glycogen consumption for a “trained endurance athlete”—and I probably don’t really qualify as “trained,” in the sense the study uses. So to survive a marathon, I have to take on supplementary energy mid-race, for which I will carry tubes of disgusting sugar syrup.

Before I started training for Portland, I didn’t pay much attention to the state of my running shoes—I bought new ones when the holes in the uppers got too obvious. That’s okay when the longest run I do is about eight miles—once my weekly schedule started including longer distances, I noticed more post-run pain when my shoes’ insoles deteriorated. I began investing in gel insole inserts and actually paying attention to how much mileage my shoes had accumulated.

The funny thing about shoes, though, is that familiarity is almost as important as adequate support. Last year I bought new shoes about a month out from the marathon—and ran some truly miserable long runs in them. Lesson learned. It turns out that a new pair of shoes takes some breaking in, especially if you switch brands, as I had. I ended up running the marathon in the shoes I’d considered shot (with new insole inserts), and felt better at the end that I had on a fifteen-mile run in the new ones. I now stick to one brand of shoes, with the same inserts if possible, and I don’t wear new shoes on a long run until I’ve worn them on a number of short ones.

Part of the reason that my running comfort is so sensitive to the quality of my shoes may be that human feet aren’t evolved to run in running shoes. Running on two legs sets humans apart from our closest evolutionary relatives, and we’ve probably been doing it for millions of years—but highly padded running shoes are a very recent invention. This is the central argument in favor of a recent fad for barefoot running [PDF]—that, once you build up some necessary calluses, running without the artificial support and padding of a running shoe is less stressful. A biomechanical comparison of barefoot and shod runners provided some of the first data to support this hypothesis earlier this year. Essentially, barefoot runners tend to land each step toes- or mid-foot-first [PDF], which absorbs the force of a foot-strike more effectively than the heel-first tread of shod runners.

I did see a few of my fellow marathon runners wearing nothing but “barefoot” running shoes like the ones pictured here, which provide protection against rough pavement but no artificial padding. I’m not going to be doing that any time soon. But maybe I’ll try to add some barefoot workouts into my training routine, if I survive Seattle and decide to run a third marathon.

I intend no endorsement of any products pictured or linked to in this post. Thanks to Conor O’Brien, who pointed me to the PLoS Computational Biology article cited above.

Based on a careful analysis of D&T visitors over the past month, I conclude that orgasms are a popular topic.

Absolute unique visitors per day, tabulated by Google Analytics..

So perhaps you folks would be interested in an entire blog carnival about orgasms? I think this is very likely. Fortunately for you, orgasm is the theme of this month’s Carnal Carnival, hosted with great enthusiasm by Scicurious. Enjoy!

“Sociable tortoises” would make a pretty good name for a band. I assume they’d be somewhere in the genre phenotype space between Vampire Weekend and The Decemberists.

Call it the “smugness threshold.” Higher income is only associated with greater emotional well-being up to a point—but past that point, people with higher incomes still report greater perceived happiness. (Neurotic Physiology)

Heads up! Tortoises follow the gaze of other tortoises, indicating unexpected social intelligence. (The Thoughtful Animal)

Who knows what a fish is thinking? Siamese fighting fish will famously attack other fish or their own mirror images with equal vigor—but their brains express different genes when looking at their own images! (NeuroDojo)

Best paleontological reconstruction illustration ever. Pterosaurs may have launched into the air by “vaulting” on their arms, not jumping with their teeny-tiny legs. (80 Beats)

Phylogenies on the witness stand. Ed Yong surveys the use of evolutionary trees as evidence in legal cases. (Not Exactly Rocket Science)

Can’t wait to see the phylogeny of the Septuagint. Texts, especially hand-copied manuscripts, mutate over time in much the same way as DNA. (The Atlantic)

We are also the beaver. New analysis of fossils identifies the sister group to Castor, the genus containing modern beavers. (Open Source Paleontologist; original article on PLoS ONE; interview with one of the authors)

That’s what undergrad field assistants are for. A classic study of bitter taste as predator deterrent had students taste-testing tadpoles. (Wonderland)

Correction, 22 December 2010: Vincent Lynch, author of the second paper discussed in this post, notes in the comments that he didn’t actually conclude that female orgasm was an adaptation. I’ve corrected the post accordingly.

Whether or not a trait is an adaptation, shaped by natural selection for a specific function, can be a surprisingly contentious question in evolutionary biology. When the trait in question belongs to human beings, though, “contentious” reaches a whole new level—because when evolutionary biologists consider humans, their conclusions get personal.

Among the myriad traits and behaviors of Homo sapiens evolutionary biologists might choose to study, few can be as personal as the female orgasm. The adaptive function of male orgasm is about as clear-cut as possible—it’s a mechanistic necessity for uniting a sperm with an egg. But while female orgasm is enjoyable (or so I am told; this is as lousy a point as any to admit that my expertise in this phenomenon is purely academic), it isn’t necessary for fertilization. No man can be a father without having had at least one orgasm, but a woman could conceivably give birth to a huge family without having any.

To explain the existence of female orgasm in an evolutionary context, then, biologists have two options: (1) discover a way in which female orgasm shapes reproductive success indirectly, or (2) conclude that female orgasm isn’t an adaptation. Possibilities advanced for the first option range from the benefits of closer bonding with a mate—sex is, after all, about more than mere reproduction—to suppositions that the contractions associated with orgasm help draw semen into a woman’s reproductive tract.

The argument in support of non-adaptive female orgasm takes a developmental perspective: that female orgasm is really male orgasm, as experienced in a female developmental context. That is, women have orgasms for the same reason men have nipples—because the anatomies of both sexes are constrained by their origins in the same underlying developmental program. If this is the case, natural selection would work to optimize male orgasm, without necessarily affecting female orgasm—and that suggests a way to test whether female orgasm is an adaptation.

Natural selection removes less-fit versions of traits from a population—making that trait less variable within the population under selection. Traits that don’t affect survival or reproductive success, on the other hand, are free to accumulate variation via mutation. So non-adaptive traits can be identified by comparing their variation to traits with known adaptive functions.

Who cares what natural selection thinks, anyway? Photo by JorgeMiente.es.

Psychologist Kim Wallen and philosopher of science Elisabeth Lloyd (who had advanced the hypothesis that female orgasm is non-adaptive in a 2005 book) made just such a comparison in a 2008 study. Variation in female orgasm would be challenging to measure, so they used the clitoris as an anatomic proxy. This let them use the penis—which shares a developmental origin with the clitoris and is presumably under natural selection associated with male sexual function—as an adaptive standard for comparison. In comparison to (flaccid) penis length, Wallen and Lloyd found that clitoris length was indeed more variable [$a]. As a second control, the authors also compared variation in clitoris and penis length to variation in the length of women’s vaginas, understanding that this trait, unlike the clitoris, is important for female reproductive success. Vaginal length turned out to be about as variable as penis length, and much less so than clitoris length.

There are several objections to be made to Wallen and Lloyd’s analysis, and many were made in a response [$a] by evolutionary biologist Vincent Lynch. Lynch objected to the use of length as the focal measure for the size of the clitoris, and showed that clitoral volume was about as variable as penile volume. (I would add that the study of social insects Wallen and Lloyd cite as a precedent for their analysis isn’t actually focused on variation, but on the symmetry of traits under consideration, which is not quite the same thing.) More critically, though, Lynch points out that there isn’t any known relationship between clitoral size and ability to achieve orgasm—so the data don’t have the bearing on the question that Wallen and Lloyd assigned in the first place. Lynch concluded that female orgasm is an adaptation after all—a more conservative interpretation of his result is that we can’t answer the question by measuring clitorises.

Understanding the evolution of human sexual behaviors can help us to figure out how best to navigate the tricky business of a sexual relationship with another person—an approach most recently exemplified in the book Sex at Dawn. But we also tend to view evidence that natural selection favors a particular trait or behavior as a kind of approval, or as evidence of what is “natural.” That’s silly. Whether or not they help to make more babies, orgasms are fun, and they’re a wonderful part of our most intimate expression of affection and love. In some respects, that’s all we need to know.

Gotta get funded to do the science. Over at dechronization, Rich Glor lays out tips on writing a doctoral dissertation improvement grant. (Part 1, Part 2, Part 3, Part 4Part 5)

Scientific support for the siesta. A daytime nap can improve memory performance. (BrainBlogger)

Hint, hint. Submissions for the Open Lab 2010 collection of online science writing close at the end of the month. (The Thoughtful Animal)

This just in. Eating fewer calories than you burn results in weight loss—even when most of those calories are in Twinkies. (Weighty Matters)

Because you can’t develop Seasonal Affective Disorder if your brain is too small. Lemur species that live in habitats with greater seasonal changes have larger brains. (NeuroDojo)

Paging Dr. Pangloss. Psychologists are surprised to discover that the sight of cooked meat makes men less aggressive. They will no doubt also be surprised to find that it makes men ask for a fork and A-1 Sauce, too. (AOL News, McGill University press release)